The following "crop circle" is not widely known, perhaps because it's relative old (1991). However the case, also known as the Milk Hill Message, is very interesting.

In 1991 American Erik Beckjord made a crop circle directed to the "ETs" reading "Talk to us". A week later it seemed an answer appeared in strange letters or glyphs. Scholars looked studied the "answer" and they came with different interpretations. Some suggest that the "answer" is of such intelligence that different messages have been encoded within, i.e. that all the interpretations of the different scholars are right.

Beckjord put the statement "talk to us", and they may have replied with:

1- The Creator wise and loving
2- I am against acts of cunning
3- I've hidden at present
4- New Breed of People

You can read more about the case here (good read):
[link to www.dcca.nl]

I cannot say that I truly believe in crop circle, but the Milk Hill Messagee is very intriguing, as many other crop circles are.